(1) Field of the Invention
The present invention relates to a novel process for producing a 3,5-di(.alpha.-methylbenzyl)salicylic acid derivative useful as a color developer for pressure-sensitive copying paper or heat-sensitive paper, as well as to a color developer containing a polyvalent-metal-modified product of the 3,5-di(.alpha.-methylbenzyl)salicylic acid derivative.
(2) Description of the Prior Art
Salicylic acid type compounds, when used as a color developer for pressure-sensitive paper or heat-sensitive paper, are generally excellent in clearness of developed color image, storage stability, etc. In the pressure-sensitive paper using a salicylic acid type compound as a color developer, however, there are problems of (a) slow color development due to the insufficient compatibility of said compound with a capsule oil used for dissolution of a color former and (b) insufficient water resistance of the developed color image, i.e. disappearance of said image by dissolution in water. Various attempts have been made in order to alleviate these problems. For example, it has been proposed to introduce an aromatic substituent into the salicylic acid skeleton.
The introduction of an aromatic substituent (e.g. a benzyl group) into, in particular, the 3- and 5-positions of the salicylic acid skeleton is effective. A color-developing sheet coated with a color developer consisting of, in particular, a polyvalent-metal-modified product of a salicylic acid compound with a benzyl group having a methyl group at the .alpha.-position, shows outstanding color developability and has excellent resistance to yellowing when exposed to sunlight, excellent stability of the developed color image and excellent resistance to yellowing by nitrogen oxides in air; therefore, the sheet is very advantageous in handling and storage. The above color developer, however, has drawbacks in that its slight solubility in water causes the disappearance of developed color image by water and its high melting point (190.degree.-220.degree. C.) makes slow the rate of its dissolution in a capsule oil containing a color precursor. This leads to inferior initial-stage color developability at low temperatures and limited use in cold districts. (i) It is known to produce a 3,5-disubstituted salicylic acid from a corresponding substituted phenol and carbon dioxide according to the Kolbe-Schmitt reaction. For example, it is disclosed in Japanese Patent Publication No. 10856/1974 that 3,5-di(.alpha.,.alpha.-dimethylbenzyl)salicylic acid is produced from 2,4-di(.alpha.,.alpha.-dimethylbenzyl)phenol obtained from phenol and .alpha.-methylstyrene. This production process, however, generally incurs a high production cost because the reaction steps are many, the yield is low, and the reaction for introducing a carboxyl group is carried out under high temperature and high pressure conditions.
(ii) In order to overcome such a problem of high production cost, it was attempted to produce a similar salicylic acid compound by, for example, subjecting salicylic acid or a salicylic acid ester to alkylation.
It is known to react 1 mole of salicylic acid with 2 moles of 1-phenylethanol to obtain 5-[.alpha.-methyl-4-(.alpha.-methyl-benzyl)-benzyl)salicylic acid or a mixture of 5-[.alpha.-methyl-4-(.alpha.-methylbenzyl)-benzyl]salicylic acid and 3,5-di(.alpha.-methylbenzyl)salicylic acid [Japanese Patent Application Kokai (Laid-Open) Nos. 100493/1986 and 96449/1987]. This production process has various problems. The substituted salicylic acid obtained is a mixture of various compounds. Therefore, a monosubstituted salicylic acid or a mixture of a monosubstituted salicylic acid and a disubstituted salicylic acid is isolated in the form of a metal salt by a complicated procedure; when the metal salt of the mixture is used as a color developer for pressure-sensitive copying paper, the resulting color-developing paper has insufficient reproducibility in properties such as color developability, storage stability and the like.
(iii) It was proposed to react salicylic acid with a styrene compound in the presence of an aliphatic carboxylic acid using an organic sulfonic acid or an inorganic acid as a catalyst, to produce a disubstituted salicylic acid [Japanese Patent Application Kokai (Laid-Open) No. 91043/1990].
This production process is not advantageous industrially because the reaction is carried out at 90.degree.-130.degree. C. using an aliphatic carboxylic acid (e.g. acetic acid, propionic acid) and sulfuric acid or methanesulfonic acid in combination, with the acid used in an amount of 50% by weight or more based on the salicylic acid and accordingly the process has a problem of disposing a large amount of the waste acid. In this process, the reaction product is, for example, a mixture of 3,5-di(.alpha.-methylbenzyl)salicylic acid with 3-.alpha.-methylbenzyl-5-(1,3-diphenylbutyl)salicylic acid and 3-(1,3-diphenylbutyl)-5-.alpha.-methylbenzylsalicylic acid. When such a reaction product is used as a color developer for pressure-sensitive paper, the resulting color-developing paper has the same problems as in the above (ii).
(iv) With regard to the alkylation of an alkyl salicylate, it is known to react, for example, methyl salicylate with styrene in the presence of an alkanesulfonic acid to produce methyl 3,5-di(.alpha.-methylbenzyl)salicylate [Japanese Patent Publication No. 26772/1986]. In this production process using styrene, various styrene polymers and other by-products are formed. This process, similarly to the process for production of salicylic acid ester resin disclosed in Japanese Patent Application Kokai(Laid-Open) No. 133780/1989 by the present inventors, is unable to suppress said reactions (resinification) even if the catalyst, the molar ratio of salicylic acid ester and styrene, the reaction temperature, etc. are varied.
In Comparative Example 1 (described later) which was conducted for confirmation of the Example 1 of the above Japanese Patent Publication No. 26772/1986, the selectivity of methyl 3,5-di(.alpha.-methylbenzyl)salicylate was 43%. This is because not only a monosubstituted compound and a trisubstituted compound but also styrene polymers, a dimer adduct, etc. are formed. Moreover, the amount of alkanesulfonic acid used is large. Therefore, the above process is not advantageous.